This invention relates to improvements in small synchronous a.c. motors, and particularly to a mechanism that insures that such motors will alway begin rotation in the same direction, but which thereafter allows reverse rotation.
Small a.c. synchronous motors are often used as the prime mover for controlling the operation of valves and other mechanisms. The motors typically have a permanent magnet rotor and a circular array of poles of alternating polarity disposed about the rotor. Such motors when energized are likely to begin rotation in either a clockwise or a counterclockwise direction. If the motor is stalled by an overload on its shaft, the motor will then reverse rotation. Most control applications require that the motor always rotate in the same direction upon being energized, and rotation in either direction will not be acceptable. To insure single direction rotation upon start up, it has been typical to use a mechanical pawl mechanism that engages with gearing connected to the motor shaft to physically prevent the shaft from rotating in the unacceptable direction while allowing it to freely rotate in the acceptable direction. Examples of mechanisms that physically block rotation except in one direction are found in U.S. Pat. No. 3,225,874 issued Dec. 28, 1965 to Woolley for "Unidirectional Self Starting Device for Synchronous Motor; U.S. Pat. No. 3,473,058 issued Oct. 14, 1969 to Landgraf, et al. for "Self-Starting Synchronous Motor", and U.S. Pat. No. 3,501,658 issued Mar. 17, 1970 to Morley for "Bidirectional Motor with Directional Control Means for Starting in Either Direction".
Mechanisms that physically block rotation of the motor except in one direction do not allow the motor to reverse direction if it is stalled by an overload on the motor shaft because the reverse direction is always blocked.
My invention provides a mechanism for insuring that the motor will always begin its rotation in the same direction upon start up but the mechanism is disengaged shortly after start up so that the motor can reverse its direction when an overload condition is encountered.